How deep do roots grow
Written by : Werner Hendriks
All tree species have their own natural habitat in which they thrive better than other species. But this does not mean that it is the best and only suitable location for this particular tree species. Each tree species has its own genetics. This results in the roots of some trees rooting deeper than others under the same conditions. So far, the strongest hypothesis for explaining the ability of certain tree species to grow deeper is the “anaerobic root growth” hypothesis in which it is assumed that all tree species can push through the soil with the same pressure. And that tree species tolerant of wet soils can grow roots during wet conditions, periods when soil strength is low, while species less tolerant of these wetter conditions cannot. So yes, there are tree species that are genetically able to root deeper than other species, but these differences are nil compared to the difference in root depth under different site conditions.
A tree species that roots only shallowly or deeply simply does not exist.
Do you know how deep tree roots really grow? There are countless publications on the root ball dimensions of wind blown down trees, despite abundant publications on the depth of tree roots in many different countries, there are few studies of mature tree root systems that have not been uprooted. Research has successfully led people to believe that tree roots have essentially a horizontal orientation. In fact, it goes so far that even many tree professionals believe that trees root only superficially and deep roots are a myth.
A tree root can grow incredibly deep.
In the paper, “On The Maximum Extent of Tree Roots,” dd 1991, E.L. Stone and P.J. Kalicz summarize several previous root depth studies. 49 genera and 211 species growing in a wide variety of soil types are noted. For example, they found numerous examples of trees that rooted roots more than 10 meters deep, and one report described a record depth of 53 meters; in rock crevices, roots can grow incredibly deep. Clearly, a tree’s ability to grow deep roots is not a limiting factor in the designing urban tree pits.
All trees can develop a deep root system if soil conditions allow it.
Although a tree’s genetic characteristic plays a role in its root development, soil conditions are critical. All tree roots are genetically capable of growing many feet deep into the ground, but the depth is greatly influenced by soil conditions. The most common limitations for our trees in urban areas are soil compaction and poor drainage. Since these conditions are quite common in urban areas, trees with shallow roots are often considered “typical. Now, more and more urban trees are provided with a engineered tree pit solution (also referred as urban tree planting system), allowing root growth under pavement.
How deep tree roots grow depends on three simple factors.
Roots need three things: water, oxygen and soil compaction levels low enough (or with voids large enough) to allow root penetration. When all these conditions are met, roots can grow to great depths. Under ideal soil and moisture conditions, roots have been shown to grow more than 10 meters deep. All three factors must be evaluated together.
1.) Soil density of the soil.
Roots cannot grow far into soil layers that have excessive bulk density, in other words, overly compacted soils. Soil compaction due to urban development and how we use the land is often the cause of root limitation for urban trees. Compaction occurs when the soil is compressed, breaking down the structure, reducing the open pore volume and increasing the strength – the physical resistance of the soil to penetration.
2.) Soil aeration
All tree roots need oxygen to breathe, but some flood-resistant species have evolved strategies to survive at reduced oxygen levels. For most tree species, when oxygen drops below 10-15% in a soil, root growth is inhibited and stops completely at 3-5%. Such conditions occur when air spaces in the soil are replaced by more soil (compaction), water or other gases such as carbon dioxide.
3.) Soil moisture
Waterlogged soils result in poor gas exchange, causing the soil to lose oxygen, leading to anaerobic conditions and subsequent root death. Soils with permanently high water tables usually cause trees to develop very shallow, widespread rooting systems. Drought conditions also cause some trees to produce shallow root systems to maximize water absorption from rain and condensation near the soil surface. If there is a deeper underground water supply, roots can make good use of it, provided the soil conditions at that depth are suitable for root penetration and oxygenation.
Now we know conditions how to get tree roots to root deeper. Or possibly shallow, you can adapt your design to these needs. Or else you can just ask me, Werner Hendriks or own of our technical design team members for help.
